reCAPTCHA is bar none the way to go. Not only is is robust, but it also serves a useful purpose---it uses user answers to the reCAPTCHA to digitize books by having users recognize text from scanned books where OCR would perform poorly.

http://www.google.com/recaptcha

If your captcha doesn't serve a useful purpose besides being a captcha, it's going to be hard to beat reCAPTCHA.